RF power amplifiers used in communication systems are generally required to have a specified degree of linearity. Circuits employing class A or class AB operated devices are controlled to be in the linear range of their characteristics and provide such linear amplification. Greater linearity can be obtained by adding RF feedback or feed forward circuitry. It is well known, however, that linearly operated devices exhibit relatively low average dc-rf efficiency. Consequently, there is an increase in power consumption and heat dissipation. To obtain higher average dc-rf efficiency, amplifiers using class C operated devices have been utilized. Class C operation uses the non-linear characteristics of the device whereby the dc-rf efficiency is greatly improved but linearity is impaired. Where envelope modulation is used, the average dc-rf efficiency of a class C amplifier can be further improved by incorporating a pulse width modulator in the class C amplifier as disclosed, for example, in U.S. Pat. No. 4,776,036 issued Oct. 4, 1988 to G. L. Hulsey et al.
While class C operation significantly improves efficiency through use of non-linear characteristics, the non-linear operation causes signal distortion and some form of distortion reduction circuitry must be added to meet linearity specifications. U.S. Pat. No. 4,276,514 issued June 30, 1981 to M. Y. Huang discloses a wideband, phase compensated amplifier with negative feedback to reduce distortion in the output signal in which a low cost, power efficient class C amplifier is positioned along a primary signal path and a feedback loop is coupled to provide negative feedback of distortion signal components in the amplified output signal. The negative feedback loop is coupled to provide the distortion signal components as negative feedback to the primary signal path by adding the distortion signal components to the input signal ahead of a first amplifier stage. The distortion reduction arrangement, however, requires a complex feedback path operating at RF frequencies with critical time delay provisions for phase adjustment.
U.S. Pat. No. 3,900,823 issued to Sokal et al Aug. 19, 1975 discloses a power amplifying and signal processing system for modulated carrier signals which separately processes the amplitude component of the system input signal and the component of frequency or phase or both frequency and phase, and later recombines the separately processed components to provide an output signal. The input signal is fed to a power amplifier whose output provides the output for the system. The input and output signals of the system are fed by separate paths to a comparator which compares those signals and emits an error signal to a controller. The controller regulates the amplitude and phase, or both, of the power amplifier's output to null the error signal. One or both of the signal paths to the comparator may have in it a non-linear function generator which acts upon the signal fed by that path to the comparator. While the system regulates the amplifier operation responsive to the modulation components, it is relatively ineffective to remove distortion at the input signal rate.
U.S. Pat. No. 4,574,248 issued Mar. 4, 1986 to K. A. Snodgrass discloses a transceiver in which the radio frequency signal output is coupled as the input to a power amplifier through a directional coupler to provide a sample of the RF input. The sampled input is coupled through a radio frequency detector to detect the envelope of the RF input and thence through a limiter to the input of a difference amplifier. The output from the power amplifier is coupled through a low pass filter to provide the amplified radio frequency output through a second directional coupler to provide a sample of that radio frequency output. The sampled output is also coupled through an RF detector to detect the envelope of the RF output and provide that of the second input to a difference amplifier. The output of the difference amplifier is then used to control the bias of the RF power amplifier for power output control. The signals representing the detected envelope of the RF input and the RF output are also compared in a difference amplifier to produce an output which is compared with a fault threshold to provide a fault signal representing a failure in the output of the control loop or power amplifier. The bias control provided by this arrangement, however, does not effect distortion correction at the signal rate.
In some communication systems, a signal input to an RF power amplifier using phase or frequency modulation also exhibits envelope variations that must be preserved in the amplifier output so that the use of a class C amplifier device with a pulse width modulator to maximize efficiency results in distortion of the output signal envelope. In cellular telephone systems, for example, the outputs of a plurality of FM or phase modulated channels are sometimes combined so that the combined outputs can be applied to a common power amplifier. The envelope of the combined channel signal exhibits significant amplitude variations which variations must be preserved in the amplifier output. In systems using phase shift keying or differential phase shift keying, it is advantageous to reduce the amplitude of the phase modulation signal during shifts between predefined phases to minimize the output signal bandwidth. As a result, the envelope of the modulated signal includes an amplitude modulation component which must be amplified without distortion. It is an object of the invention to provide an improved RF amplifier for envelope and phase varying information signals having both high efficiency and high linearity.